1. Field of the Invention
Exemplary aspects of the present invention generally relate to an optical sensor and an electrophotographic image forming apparatus using the same, and more particularly, to an optical sensor for detecting image information and an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile, and a multi-function machine, using the same.
2. Description of the Background Art
Various types of optical sensors for detecting image information used in an electrophotographic image forming apparatus using dry toner, and technologies relating to the image forming apparatus equipped with such an optical sensor, have been proposed.
A typical example of a related-art optical sensor is that disclosed in Unexamined Utility Model Application Publication Hei 02-111162, which proposes a shutter device for an optical detection element that is included in a toner density control unit that regulates the toner density of a developing unit by enabling the optical detection element to detect light reflected from a visible image formed on a photoreceptor. The shutter device includes a shutter member that is disposed between the optical detection element and the photoreceptor and protects a surface of the optical detection element from contamination. The shutter member includes a window reciprocally movable in predetermined directions so that a light emitting area and a light receiving area of the optical detection element can be opened and closed. The shutter device includes a calibration plate disposed on a rear surface of the shutter member facing the light emitting area and the light receiving area, when the window is at a closed position.
One example of the optical sensor utilized in an image forming apparatus that uses electrophotography to form an image, and which requires calibration of its sensitivity (hereinafter “sensitivity calibration” or simply “calibration”), is a toner adhesion sensor for measuring an amount of toner adhesion. Such an optical sensor is normally an analogue sensor that measures the amount of toner adhesion on the basis of a voltage value. Calibration is needed because the amount of light of the light emitting element (for example, LED) may fluctuate over time due to deterioration of the, causing an analogue output value to fluctuate.
The toner adhesion sensor measures the amount of toner adhesion by irradiating a toner image formed on a toner image bearing member such as a photoreceptor and an intermediate transfer belt with light and detecting the amount of the reflected light.
There are two types of known methods for measuring an amount of toner adhesion. One method uses specular light, that is, specularly reflected light, and another method uses diffusely reflected light. The specular light is used to measure mainly an amount of black toner adhesion. The diffusely reflected light is used to measure mainly an amount of color toner adhesion.
The detection principle of the method using the specular light involves detecting a state of the specular light specularly reflected from the surface of the toner image bearing member through the toner image formed on the toner image bearing member. As a result, however, the intensity of the specular light is weakened.
The detection principle of the method using the diffusely reflected light involves detecting an intensity of the diffusely reflected light reflected by the toner image itself formed on the toner image bearing member.
When there is no toner image on the toner image bearing member, an output value of the sensor using the specular light is at its greatest, whereas the output value of the sensor using the diffusely reflected light is approximately zero. Such a difference in the output value may determine the operation of the sensitivity calibration in the image forming apparatus.
Accordingly, when the surface of the toner image bearing member has a surface similar to the specular surface of the photoreceptor, for example, the sensor using the specular light can calibrate the sensitivity using the specular light specularly reflected by the surface of the toner image bearing surface. For example, when the sensor detects the surface of the toner image bearing surface, the light emission intensity of the light emitting element is adjusted such that the output value becomes, for example, 4V.
On the other hand, in the sensor using the diffusely reflected light, the output value may be approximately zero relative to the surface of the toner image bearing member, making calibration impossible. Calibration requires adjusting the intensity of light emission by the light emitting element such that a reference voltage is output when the sensor detects a reference surface. However, when an output voltage is almost zero, it is hardly possible to adjust the intensity of light emission of the light emitting element.
As described above, when the surface of the toner image bearing member is substantially similar to the specular surface as commonly implemented in the image forming apparatuses, the sensor using the diffusely reflected light cannot calibrate sensitivity using the surface of the toner image bearing member, and therefore requires a reflecting plate for calibration.
In a case in which the sensor detecting the specular light and the sensor detecting the diffusely reflected light are both provided in one sensor, the calibration result of the sensor detecting the specular light can be taken into account in the sensor detecting the diffusely reflected light by calculation. However, such an arrangement increases a data processing load.
Ideally, the reflecting plate for calibration should be disposed on the surface of the toner image bearing member, such as the photoreceptor and the intermediate transfer member to be detected because the surface of the toner image bearing member is a surface to be detected. However, disposing the reflecting plate on the surface of the toner image bearing member may disrupt image formation. Therefore, a method for disposing the reflecting plate for calibration on the sensor unit side is known.
For example, according to the Unexamined Utility Model Application Publication Hei 02-111162 described above, the shutter is provided to the detection surface side of the optical detection element (the toner adhesion sensor), and the calibration plate is disposed on the back of the shutter. Upon closing the shutter, the calibration plate may perform the sensitivity calibration.
However, because the calibration plate is disposed on the back of the shutter which is a moving member, positional reproducibility of the calibration plate may be poor due to the open-close movement of the calibration plate.
The output of the sensor is very sensitive to the distance to the object to detect. Thus, it is not preferable to dispose the calibration plate on the back of a device which moves repeatedly and has poor positional reproducibility.
According to Japanese Patent No. 3661446, the reference reflection surface (the equivalent of the reflecting plate for calibration) is disposed in the sensor (not the shutter), and fixed optical paths for calibration are provided between the reference reflection surface and the light emitting element, and between the reference reflection surface and the light receiving element, respectively.
According to this configuration, while normal detection is performed, the reflected light from the reference reflection surface enters the light receiving element through the fixed optical path for calibration, and the effect of the light passing through the fixed optical path for calibration is subtracted upon normal detection.
However, there may be a drawback to the foregoing configuration in that the output for calibration may be superimposed on the output for detection, thus shrinking the dynamic range detection output. Furthermore, every time normal detection is performed extra calculations need to be performed, thereby increasing the data processing load.
In view of the above, when normal detection is performed, it is desirable that the output value for calibration obtained from the reflected light from the reference reflection surface is not superimposed.
Another known arrangement is disclosed in Japanese Patent Laid-Open Application Publication 2002-268314, in which the reference reflection plate is provided in the vicinity of the light emitting element for the diffuse reflection in the optical sensor including two light emitting elements and a single light receiving element. A first optical path through which the reflected light from the measurement point enters the light receiving element, and a second optical path through which the reflected light by the reference reflection plate enters the light receiving element are also provided. An optical path intercepting member which intercepts either the first or the second optical path and opens the other path is provided.
The optical path intercepting member is driven by a solenoid disposed in the sensor housing. Although not explicitly indicated in Japanese Patent Laid-Open Application Publication 2002-268314, a contamination prevention shutter disposed in front of the sensor is driven to open and close by the solenoid in the sensor. As described above, when the driving force of the single solenoid is provided to the optical path intercepting member in the optical sensor and the shutter disposed outside the shutter, it is cost-effective. However, installation space for the solenoid and a mechanism to transmit its driving force requires space, increasing the size of the sensor unit.